Various approaches to making a chair seat and/or back form fitting for various users are known in the industries of seating manufacture. These approaches range from the rather traditional use of contouring synthetic foam, to seat/back shells that have a degree of flex. There have also been approaches that use a frame that has a membrane or sling stretched or supported across or within a frame. Problems can arise from each of these approaches.
For example, under normal manufacturing conditions, it can be difficult to vary the amount of firmness and corresponding support in different areas of a foam padded cushion. Additionally, foam can lead to excessive heat-build-up between the seating surface and the occupant. One of the problems with foam is the forming and molding process. Current manufacturing technology makes it a relatively inefficient process compared with the manufacture of the other components that make up a chair or seating surface. Often, the forming/molding of a contoured seating surface can be slow, thereby requiring the manufacturer to make several molds (typically hand filled) in order to maintain an efficient level of production.
Another problem inherent to the use of foam is that in order to achieve a finished look, the cushions typically must be covered, e.g. upholstered. When a manufacturer upholsters a cushion, a number of issues may arise. For example, the formed or molded foam may have curves, many of which can be compound-curves, which leads a manufacturer to use glue or other adhesives to make the fabric conform to the contours. This laminating technique often makes the foams surface firmer than it was when it was originally molded/formed because the glue/adhesive and the fabric are now part of the foam structure. Additionally, the amount of change in firmness can vary from fabric to fabric which results in an unpredictability of the firmness of a cushion from one manufactured unit to the next.
Alternatively, if a slipcover is used, it must be sized properly. Such sizing can be difficult as a result of the differing mechanical properties found from one fabric to another. The most important properties of a fabric when upholstering a contoured surface are its thickness and its rate of stretch. Thickness variations can make one fabric upholster smooth around radii or contours, while a thicker one will wrinkle in the same area. Variations in the amount of stretch can lead to other problems. Therefore, a proper size slipcover in one type of fabric, with its stretch characteristics, may be the wrong size in another type or style of fabric. Often a manufacturer will “wrap” a piece of fabric around a cushion and then staple the fabric to the underside/backside of the cushion. This approach also suffers from the aforementioned problems associated with using variable fabrics. Additionally, the manufacturer must now cover the staples and the area of the cushion not covered by fabric in order to achieve a finished look. This leads to an additional manufacturing step or molding etc. that often also has to be upholstered.
The other reality of cushion upholstery, regardless of the techniques used, is that whether it is done in a small shop or in a production situation, it can be the most labor-intensive aspect of chair/seating construction.
In the case of incorporating flex into the shells of a chair, it can be difficult to achieve the proper amount of flex in the right areas to give correct ergonomic comfort for a wide range of individuals. In the case of a membrane approach, the curves imparted on the membrane by the frame are often simple in nature (non-compound) and thus cannot provide the proper contouring necessary for ergonomic comfort. Also, this approach can lead to “hammocking,” where the areas adjacent a pressed area have the tendency of folding inward, squeezing the occupant, and not yielding the proper ergonomic curvatures. An additional problem with membrane chairs is that the tension of the membrane may not be appropriate for all ranges of users.
To solve some of these problems, manufacturers have produced “sized” (i.e. small, medium and large) chairs that effectively narrow the amount of contouring-compromise that the designer must normally exercise. This approach, however, may require the manufacturer to tool three independent products instead of one, and the manufacturers, wholesalers, and retailers having to stock (in this example) three times the quantity of product. Additionally, the purchaser ends up with a chair that at some point in the future may be the wrong size for a different user.
In some seating structures, the frame members, such as a backrest support, may be made from metal to accommodate the large loads applied thereto by the user. Metal, however, can be expensive to purchase as a raw material, as well as to form into a final product. Moreover, the resultant chair is relatively heavy, leading to increased shipping costs and decreased portability. In some cases, various components have been made of plastic or composite materials, e.g., fiberglass. These components, however, can be susceptible to wear and often cannot carry the necessary loads, for example in bearing.